Hydrofluoric acid is useful in such diverse fields as isoparaffin-olefin alkylation, fluorination, semiconductor manufacture, steriod synthesis, tantalum recovery, and xylene separation.
Industrial isoparaffin-olefin alkylation processes have historically used concentrated hydrofluoric acid catalysts under relatively low temperature conditions. The acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. As used herein, the term "concentrated hydrofluoric acid" refers to an essentially anhydrous liquid containing at least about 85 weight percent HF.
Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing a so-called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also to its sensitivity to octane-enhancing additives. For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986).
Recently, more stringent environmental regulations have prompted a new look at methods of storing and processing hydrofluoric acid. Specifically, researchers have investigated possible solvents which could be used to dilute the hydrofluoric acid (thus rendering it safer) while preserving its commercial useful characteristics. Tetrahydrothiophene-1,1-dioxide (also referred to herein as sulfolane) has been found to be a useful additive for hydrofluoric acid in isoparaffin-olefin alkylation.
Dilute solutions of water and hydrofluoric acid are highly corrosive toward carbon steel. Neat hydrofluoric acid is essentially noncorrosive toward carbon steel, and it is industry practice to handle and store neat hydrofluoric acid using carbon steel equipment. Neat tetrahydrothiophene-1,1-dioxide (sulfolane) is similarly relatively noncorrosive toward carbon steel. Surprisingly, mixtures of hydrofluoric acid and tetrahydrothiophene-1,1-dioxide are highly corrovive. Carbon steel process equipment would has a projected useful life of no more than a few months in the presence of mixtures of hydrofluoric acid and tetrahydrothiophene-1,1-dioxide.
Diluting HF with tetrahydrothiophene-1,1-dioxide overcomes the fuming tendency of the HF and makes handling and storing the HF both easier and safer. Further, even if the mixture is accidentally released from its containment facility, the HF tends to remain in the liquid solution rather than to form a dense vapor cloud.
Thus while mixtures of HF and tetrahydrothiophene-1,1-dioxide are safer than neat HF in the event of an accidental release, the mixture cannot be stored in carbon steel without extensive corrosion control measures. Clearly, then, it would be desirable to provide an economical additive which decreases the corrosion rate of carbon steel in the presence of mixtures of HF and tetrahydrothiophene-1,1-dioxide.